According to the communication protocol based on the current V.34 Recommendation and T.30 Recommendation ANNEX-F, modems operate in full-duplex mode in a control channel where communication procedures are exchanged and operate in half-duplex mode in a primary channel to transmit image information. FIG.5 shows the communication protocol.
Phase 1 is a section where a CM signal (calling menu signal) and JM signal (common menu signal) are exchanged. This section selects a modulation mode available to the calling and answer modems.
Phase 2 is a section called "line probing." L1 and L2 are signals to probe the line characteristic viewed from the answer modem each including 21 single-frequencies from 150 Hz to 3750 Hz. INFO is an information signal indicating communication capacity and signals A, A bar, B, and B bar indicate the ACK (Acknowledge) signals that receive INFO and are signals to adjust the transmit/receive timing of signal L1 and signal L2.
Phase 3 is a primary channel preparation stage which transmits V.34 image signal data which will be issued in a later stage and corresponds to the period of transmission of a long training signal (long synchronization signal). The frequency band (or symbol rate) used in this long training signal is determined based on the line characteristic probed by the L1 and L2 signals in phase 2.
The control channel includes two groups of signals; first-half section A and second-half section B. Section A is mainly for exchanging operating parameters of the modem and it is in this section that the transmission rate of the image signal data transmitted through the primary channel which follows the control channel is determined. Section B is a section to exchange control information as the facsimile terminal and it is in this section that control commands such as DIS (Digital Identification Signal) and DCS (Digital Command Signal) described in the T.30 Recommendation are exchanged.
An MPh signal in section A contains a bit (bit 50 in MPh) which determines whether or not to accept asymmetric transmission rates of 1200 bps and 2400 bps as the transmission rate of section B. The MPh signal also includes another bit (bit 27 in MPh) which requests the answer modem to transmit section B at either 1200 bps or 2400 bps.
If bit 50 in MPh of both the transmitter and receiver is "1", communication is carried out at an asynchronous transmission rate and if bit 50 in MPh of either the transmitter or receiver is "0", communication is carried out at the same transmission rate.
For both 1200 bps and 2400 bps, the control channel at the calling modem uses a 1200 Hz carrier and a band of 1800 Hz or lower, while the control channel at the answer modem uses a 2400 Hz carrier and a band of 1800 Hz or higher.
However, the T. 30 ANNEX-F currently does not accept asymmetric communications (communications carried out at different transmission rates between the calling and answer modems) in FAX transmissions and stipulates that MPh bit 50 is set to "0".
When a symmetric rate transmission is selected, if the requested transmission rate declared by bit 27 in the MPh signal differs between the calling and answer modems, it is stipulated that the transmitter and receiver should carry out communications in section B according to the slower transmission side.
In a current FAX which incorporates a V.34 modem, the transmission rate of section B is preset to either 1200 bps or 2400 bps and it is possible to select either one according to the line situation.
The primary channel contains facsimile image information data and a short training signal (short synchronization signal) which precedes the facsimile image information data. The data signal rate of this image information part is determined by the exchange of the aforementioned MPh signal of section A of the control channel.
FIG. 6 is a section diagram of a conventional receive circuit that receives facsimile control signals. A receive signal passes through AGC (Automatic Gain Control) circuit 1, A/D converter 2 and is input to QAM demodulator 3, 1200 Hz detection circuit 4, 2400 Hz detection circuit 5 and FFT circuit 6.
QAM demodulator 3 is a QAM (Quadrature Amplitude Modulation) demodulator which detects the modem performance of phase 2, line probing result and INFOoc and INFOoa, etc. of an INFO sequence used to exchange data mode (image information transmission mode) modulation parameters, and demodulates them.
1200 Hz detection circuit 4 and 2400 Hz detection circuit 5 detect 1200 Hz and 2400 Hz signals to capture signals in section B and section A in phase 2.
FFT circuit 6 is a fast Fourier transformation circuit which analyzes the frequency characteristic of line probing signals L1 and L2 in phase 2 and determines the symbol rate (frequency band) of a primary channel signal used in phase 3 and thereafter.
However, the conventional technology above has the following problems.
The transmitter transmits image information after selecting the optimum data signal rate according to the line situation by evaluating the line characteristic using the line probing signal. The transmitter fixes the data transmission rate in control channel section B at either 1200 bps or 2400 bps set beforehand.
In the case of 1200 bps, noise resistance is considerably high to such an extent that it can be an excessive quality for normal lines, whereas its data transmission rate is low. On the other hand, with 2400 bps, the opposite is the case. Therefore, transmission of the control channel with a noisy line often ends up repeating data retransmission. The difference of noise resistance between 1200 bps and 2400 bps communications of the control channel is approximately 7 dB.
In communications of the control channel above, the symbol transmission rate is as low as 600 baud, and thus it has a strong resistance in line link characteristics.
However, since band separation type full-duplex communications are used for the control channel, the receive signal level in either a high band of 1800 Hz or higher or a low band of less than 1800 Hz remains as low as close to -43 dBm specified in the T.4 Recommendation. On the other hand, a signal transmitted by the calling modem may be reflected from the exchange and introduced to the demodulator as an echo. Since the gain of this echo signal may be greater than the receive signal level, either the calling modem or answer modem will have lower reception noise resistance. Therefore, in the case of a transmission rate of 2400 bps, there is a problem of extremely high probability of bit errors in receive data of the receiver.